This invention relates to a switch for a dynamoelectric machine, and more particularly to a starting switch for a fractional horsepower induction electric motor.
Typically, capacitor start and split phase induction motors have a run winding and a starting winding placed in winding receiving slots provided in the bore of the stator core of the motor. The starting winding is energized during start up of the motor (or when the speed of the motor falls below a specified operating speed) so as to create a rotating field in the stator and to apply sufficient torque to the rotor for starting purposes. However, once the motor has accelerated to a desired operating speed, (e.g., this speed may be about 80 percent or more of the normal operating speed of the motor), the rotor is able to follow the alternations of the magnetic field created by the run windings, and the starting winding no longer is needed. Typically, the starting winding is not intended for continuous use and may fail if not deenergized during normal operation of the motor. As is conventional, a switch, referred to as a motor starting switch, is provided in the motor for energizing the starting winding only during start up of the motor and for deenergizing the starting winding once the motor has attained its desired operating speed. These motor starting switches are conventionally actuated by a centrifugal actuator mounted on and rotatable with the rotor shaft of the motor, the centrifugal actuator being responsive to the speed of the motor for actuating the starting switch from its start to its run position in response to the motor attaining a predetermined operating speeed. Centrifugal actuators typically include an actuator member movable axially on the rotor shaft from an off or stop position when the motor is stopped (or is operating below a specified operating speed) to a run position upon the motor accelerating to a predetermined operating speed. Typically, an actuator linkage operatively interconnects the motor starting switch and the actuator member of the centrifugal actuator. As shown in the co-assigned U.S. Pat. No. 4,034,173, this linkage may include a lever interconnecting the switch and actuator member, a spring, and means for adjusting the lever with respect to the actuator. In some instances, the lever is pivoted on a portion of the motor itself, for example on the end shield of the motor. These parts must of necessity be installed before the motor starting switch has been installed in the motor during manufacture thereof.
Because the actuator member of the centrifugal actuator moves only a limited distance between its off and run positions, it has heretofore been necessary to accurately adjust the relative positions of the motor starting switch and the centrifugal actuator so as to ensure that the former is properly actuated by the centrifugal actuator at a specified motor operating speed. This adjustment of the centrifugal actuator with respect to the motor starting switch is a time consuming operation and thus results at least in part in higher labor costs in manufacture of the motor. Typically, a centrifugal actuator only exerts a relatively light force on the linkage interconnecting the actuator member and the motor starting switch. With prior linkage arrangements, the force exerted on the linkage was, in some instances, too low to actuate the motor starting switch so as to energize the starting winding of the motor upon subsequent start up of the motor. This condition is sometimes referred to as a "stuck" actuator condition and it prevents the motor from starting.
During the service life of a motor, end play (i.e., axial movement) of the rotor shaft with respect to the end shields (or the frame) of the motor may develop. This end play may be sufficient so as to appreciably change the relative position of the centrifugal actuator mounted on the rotor shaft and the motor starting switch rigidly mounted on the frame or end shield of the motor thus effecting operation of the motor starting switch. Under certain end play conditions, the centrifugal actuator could fail to deenergize the starting winding upon the motor attaining its desired operating speed thus causing the starting winding to fail. Under other end play conditions, the centrifugal actuator may fail to reset the motor starting switch to energize the starting winding upon subsequent start up of the motor and thus the motor would fail to start.
Also, in certain motor applications (e.g., in a clothes dryer) in which the motor starting switch is exposed to lint-laden air, the deposit of the lint within the switch has heretofor been a problem. More specifically, the motor starting switch is typically installed in a location within the motor proximate the motor bearing lubrication system and oil vapor is oftentimes deposited on all of the exterior surfaces of the switch including the actuating plunger. Typically, the actuator plunger of prior art motor starting switches was moveable in axial direction in and out of the starting housing upon actuation of the switch, and lint on the plunger was carried into the inside of the switch housing. Also, prior art motor starting switches have oftentimes had a large gap between the plunger and the housing through which lint could readily enter the housing. Upon entering the housing, this lint would collect on various electrical components within the switch, and could, on occasion, prevent good contact between the electrical contacts within the switch thus causing intermittent or improper operation of the switch, and could, in certain instances, result in failure of the motor to start.
Also, many prior art motor starting switches had a problem of overstressing (and thus permanently deforming) one or more of the switch arms within the starting switch upon the moveable switch arm engaging a fixed contact prior to completion of the stroke of the centrifugal actuator. Typically, the force of the centrifugal actuator was such, that once the switch arm became stoppd against a fixed electrical contact, then the centrifugal actuator could continue to apply increased force to the switch arm which could drastically increase the bending moment on the switch arm. In certain applications it has been found that this overstressing was sufficient to cause permanent deformation of the switch arm and thus the switch arm lost its resilient characteristics which biased it toward contact with an opposed fixed contact when the motor was running.
Among the several objects and features of the present invention may be noted the provision of a motor starting switch operable by a centrifugal actuator which requires a relatively low force to actuate it;
The provision of such a motor starting switch which is relatively insensitive to end play of the rotor shaft and which need not be accurately positioned with respect to the centrifugal actuator during manufacture of the motor;
The provision of such a motor starting switch in which the force applied to the contacts of the starting switch by the centrifugal actuator is limited so as to prevent damage to the flexible switch arms carrying the moveable contacts;
The provision of such a starting switch which substantially reduces the entrance of lint and other airborne particles into the interior of the switch; and
The provision of such a starting switch which is of compact size and rugged construction, which is reliable in operation, which is easy to install in the motor, and which has a relatively long service life.
Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.